Coralloid Fe₃O₄ nanoclusters stacked by nanosheets are successfully synthesized using ferric nitrate as a precursor with an ionic liquid-assisted solvothermal method. For comparison, spherical Fe₃O₄ nanoclusters assembled by nanospheres are synthesized in the absence of ILs. The morphologies and structures of the products are further characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray power diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results indicate that the ionic liquid 1-n-decyl-3-methylimidazolium chloride ([Dmim]Cl) plays an important role in controlling the growth orientation of building blocks via selectively adsorbing on a specific crystal plane. A formation mechanism has been proposed based on the interaction between ILs and Fe₃O₄ crystal planes. The catalytic activity of nanoclusters has been evaluated with the Fenton reaction under UVA irradiation. The coralloid nanoclusters show the highest catalytic activity for the degradation of phenol due to their special morphology and unique surface properties. The synthesis strategy may provide a new way to construct nanomaterials with novel morphologies and excellent properties.